1. Field of the Invention
The present invention relates to a novel electrolysis for producing an alkali metal hydroxide at high yield and high current efficiency for a long time in an electrolysis of an aqueous solution of an alkali metal chloride with a cation exchange membrane. More particularly, it relates to an electrolysis for producing chloride in an anode compartment and hydrogen and an alkali metal hydroxide in a cathode compartment by partitioning the anode compartment from the cathode compartment with a cation exchange membrane and feeding an aqueous solution of an alkali metal chloride into the anode compartment, especially it relates to an electrolysis of an aqueous solution of sodium chloride to produce sodium hydroxide at high efficiency for a long time.
2. DESCRIPTION OF THE PRIOR ART
It has been known for a long time to electrolyze an aqueous solution of an alkali metal chloride with a selective cation exchange membrane. Recently, it has been proposed to electrolyze an aqueous solution of sodium chloride in an industrial scale with a cation exchange membrane made of a fluorinated resin.
in electrolysis with an ion-exchange membrane, the anode compartment and the cathode compartment are partitioned with the ion-exchange membrane and an aqueous solution of sodium chloride is fed into the anode compartment and an aqueous solution of sodium hydroxide is fed into the cathode compartment and the electrolysis is carried thereby producing chlorine in the anode compartment and hydrogen and sodium hydroxide in the cathode compartment. In accordance with the process of the present invention the content of sodium the chloride in sodium hydroxide solution can be substantially decreased. In conventional electrolysis using asbestos as a diaphragm the content of the sodium chloride in sodium hydroxide solution has been too large for use in certain fields. This disadvantage has been overcome.
At the beginning of the development of the electrolysis of sodium chloride by the ion-exchange membrane method, a cation exchange membrane having pendant sulfonic acid groups obtained by hydrolysis of a membrane made of a copolymer of perfluorocarbon sulfonyl fluoride and tetrafluoroethylene was used as the membrane.
In the electrolysis using such ion-exchange membrane, however, hydrophilic property of the sulfonic acid groups is too high whereby control of hydroxyl ions which undergo reverse diffusion from the cathode compartment to the anode compartment is too weak. When the concentration of the alkali metal hydroxide obtained from the cathode compartment is increased, the current efficiency is decreased. In the electrolysis of an aqueous solution of sodium chloride, when sodium hydroxide having a concentration of higher than 20 wt.% is obtained, the current efficiency is substantially low. The application of said method to industry has been disadvantageously difficult.
It has been proposed to use an improved sulfonic acid type cation exchange membrane having different ion-exchange capacity of the sulfonic acid groups at the anode side from that of the cathode side, that is, having a smaller ion-exchange capacity at the cathode side than that of the anode side. When such membrane is used, it is difficult to produce sodium hydroxide having high concentration. Only a small number of industrial operations for producing sodium hydroxide having a concentration of 10 to 20% have been carried out.
In order to overcome the fatal disadvantages for producing sodium hydroxide having high concentration which is caused by high hydrophilic property of sulfonic acid groups of the cation exchange membrane having pendant sulfonic acid groups, it has been proposed to use a cation exchange membrane made of fluorocarbon polymer of a layer having sulfonamide groups as the ion-exchange groups formed by treating the cathode side surface of the membrane having sulfonic acid groups with ethylenediamine, in an electrolysis of an aqueous solution of an alkali metal chloride as disclosed in Japanese Unexamined Patent Publication No. 92339/1975 and 96987/1977. It has been also proposed to use a cation exchange membrane made of perfluorocarbon polymer having sulfonic acid groups and carboxylic acid groups as disclosed in Japanese Unexamined Patent Publication No. 37198/1978.
When an electrolysis is carried out by using the cation exchange membrane made of perfluorocarbon polymer having weak cation exchange groups such as sulfonamide groups and carboxylic acid groups, the activity for reducing hydroxyl ions which undergo reverse diffusion from the cathode compartment is larger than that of the membrane having sulfonic acid groups, whereby an alkali metal hydroxide solution having high concentration can be obtained at high current efficiency.
The membrane having sulfonamide groups or carboxylic acid groups has the above-mentioned advantages whereas it has a disadvantage of high electric resistance as a low electric conductivity. It is usual to form the layer having said weak acidic cation exchange groups at the cathode side of said cation exchange membrane and to form the layer having sulfonic acid groups as main ion-exchange groups at the anode side.
It is also known that a cation exchange membrane formed by fabricating a terpolymer of CF.sub.2 .dbd.CF.sub.2, CF.sub.2 .dbd.CF--O--CF.sub.3 and CF.sub.2 .dbd.CF--O--CF.sub.2).sub.3 COOCH.sub.3 and hydrolyzing it to form carboxylic acid groups is a desired membrane for producing an alkali metal hydroxide having high concentration at high current efficiency.
The inventors have studied an electrolysis of an aqueous solution of sodium chloride by using the conventional cation exchange membrane having weak cation exchange groups such as sulfonamide groups or carboxylic acid groups at the cathode side. As a result, serious problems have been found because even though an alkali metal hydroxide having a high concentration such as 20 to 35% can be produced at a high current efficiency of higher than 90% by using such cation exchange membrane (this fact could not be expected by using the cation exchange membrane having sulfonic acid groups), the high current efficiency gradually diminishes during long periods of operation.
The descrease of a current efficiency is the serious problem since it causes a decrease in production of sodium hydroxide and the balance of movement of ions in the electrolytic cell is changed which causes problems in the operation of the electrolytic cell such as the control of the concentration of sodium hydroxide in the cathode compartment, pH control in the anode compartment, control of by-product chlorate production and control of oxygen concentration in chlorine, as considered by a person skilled in the art.